HK1098942A - Latex bonded acquisition layer having pressure insensitive liquid handling properties - Google Patents

Description

Latex bonded acquisition layer with pressure sensitive liquid handling properties

Technical Field

The present invention relates to absorbent articles such as disposable diapers, sanitary napkins and panty liners. More particularly, the present invention relates to acquisition layers and materials for such acquisition layers, which are well suited for use with thin articles comprising a high content of superabsorbent polymer material.

Background

Disposable absorbent articles are ubiquitous and are used by consumers to their high performance for acquiring and storing menses (in the case of sanitary napkins or panty liners) or for acquiring and storing urine and feces (in the case of disposable diapers, for example). However, consumers not only desire superior absorption properties, but also place increasing importance on the wearing comfort of such articles, i.e. on the dryness of such articles.

Typically, such articles include a plurality of absorbent members, at least one member being designed primarily for storing liquid and/or at least one other member being designed primarily for acquiring and diffusing liquid.

At least the storage member will typically comprise superabsorbent material mixed with the pulp fibre material normally used. Such superabsorbent materials can absorb many times their own weight of fluid (e.g., 10, 20, or 30 times) and are therefore extremely useful in designing articles with improved fluid handling properties. Many recent products employ higher and higher amounts of superabsorbent material, i.e. amounts exceeding 50% of the total weight of the storage member. These products achieve high absorption capacity with a very thin storage member and thus typically the article is overall thin. While superabsorbent materials can store large amounts of liquid, they are generally not capable of spreading liquid from the point of impact to more remote areas and are not capable of acquiring liquid as quickly as the article receives it.

For this purpose, acquisition members are used which are responsible for the intermittent acquisition of large quantities of liquid and usually also take into account the spreading of the liquid. Thus, the acquisition member plays a key role in utilizing the overall absorbent capacity provided by the storage member.

An exemplary recent absorbent article, i.e. a sanitary napkin, is disclosed in WO 00/51651. The so-called "secondary topsheet" is primarily responsible for acquisition and distribution of liquids. The storage member is provided in the form of a core capable of containing a large amount of superabsorbent polymer material.

The art of absorbent articles in the form of disposable diapers is disclosed in the following co-pending patent applications: EP application No. 02017516.2 (attorney docket No. CM2662MQ) and EP application No. 03002677.7 (attorney docket No. CM2687 FQ). The storage member of these articles may comprise more than 50% by weight of superabsorbent polymer material, even more than 80% and in certain embodiments even about 100% of superabsorbent polymer material. Although these articles are designed to have excellent fluid handling and fluid storage properties, they are sometimes not as comfortable as some consumers would like, i.e., those interested in premium products.

To obtain excellent fluid handling and fluid storage properties, the respective storage member uses a superabsorbent material, which is basically selected with an emphasis on fluid handling properties. For example, EP 304319B 1(Goldman et al) discloses the benefit of a narrower particle size distribution of superabsorbent polymers. The corresponding article will preferably not comprise many fine particles but rather coarser particles. EP 752892B 1(Goldman et al) discloses an absorbent structure which utilizes superabsorbent particles in an amount exceeding 60% and suggests the use of superabsorbent particles having a relatively high porosity.

Materials suitable for fulfilling the above general requirements of a liquid acquisition layer must fulfill these requirements not only under standard or ideal conditions, but also under a variety of conditions, i.e. at different temperatures and pressures.

Such as those encountered under conditions of use. In use, the temperature at which the acquisition layer must operate depends on both the body temperature of the wearer and the ambient temperature (which is typically below body temperature, but may also exceed body temperature under certain climatic conditions). Most of the pressure on the liquid acquisition layer is typically exerted when the wearer is sitting. The pressure depends on the weight of the wearer but to a large extent also on the surface on which the wearer sits.

Other relevant conditions relating to the properties of the liquid acquisition layer are those of storage and transport. In one aspect, this includes the conditions under which the diaper is stored and transported after manufacture. Typically, the diaper package exerts some pressure on the diaper, and depending largely on the country of sale, the diaper package may be subjected to a considerable range of temperature changes before being handed to the consumer. It is important that the liquid acquisition layer is independent of its storage and transportation history and reverts to a state that is accessible for optimal liquid handling properties. In a second aspect, the transport and storage conditions of the material to be made into the liquid acquisition layer are also relevant prior to diaper processing. Typically, these materials are provided in roll stock. The material is wound into rolls under pressure, taking into account the economics of handling and transportation. The respective volumes may also be stored for a longer period of time and exposed to a large temperature differential ambient temperature during storage or transport. Of course, also irrespective of the storage and transport history to which the material has been subjected at these early stages of its life, the material must be reconstituted such that optimum liquid handling properties can be achieved when the material is actually used as a liquid acquisition layer and intended for use in a diaper.

US 5,997,980(Matoba et al) discloses the use of hollow polyester fibers having a large hollow volume of 40% to 85% to provide a nonwoven material. The material is said to have a high resistance to compression and a high recovery from compression.

EP 1032345B 1 discloses a fluid acquisition-transfer layer for an absorbent article comprising a blend of heat-shrinkable, spiralled, thermoplastic multi-component (preferably bi-component) functional fibers. WO 03/048440(Brown et al) discloses the use of similar fibers, i.e. single polymer, spirally crimped fibers, in nonwoven fabrics for personal care absorbent articles.

WO 98/22279(Flohr et al) discloses an acquisition layer for improved liquid handling in absorbent articles. Fibrous materials, preferably carded fibrous materials, are disclosed as preferred fluid treatment materials. These fibers are chemically bonded with a resin. The disclosed resins are more hydrophilic than the polymer matrix and are non-uniformly distributed throughout the thickness direction (z-direction) of the fluid treatment material. The fluid handling material is designed to achieve an acquisition performance of less than 2 seconds for a third impact gush of liquid. The disclosed acquisition layer materials are said to be easy to handle and easy to transport without requiring large volumes.

EP 149880 (Kwok) discloses nonwoven webs of synthetic fibers consolidated by the process of carboxylating styrene-butadiene latexes, and disposable articles made therefrom. The resulting nonwoven fabric is described as having improved wet tensile and elongation properties.

US 2003/0105190(Diehl et al) discloses a latex application for use as a binder for nonwovens and a process for preparing such a latex. The latex is essentially of the styrene-butadiene type.

It is an object of the present invention to provide an absorbent article having improved liquid handling properties compared to the articles disclosed above.

Furthermore, it is an object of the present invention to provide a fluid acquisition layer for improved liquid handling properties, i.e. for use in combination with a storage layer having a high content of superabsorbent polymer.

In a more important aspect, it is an object of the present invention to provide an article which is more comfortable to wear and especially provides excellent dryness.

Disclosure of Invention

The present invention relates to absorbent articles such as diapers and sanitary napkins and acquisition layers for such articles. More particularly, the present invention relates to an acquisition layer for an absorbent article, said liquid acquisition layer comprising a plurality of fibers and a binder, the thickness of the liquid acquisition layer decreasing with external pressure and increasing when said external pressure is removed. According to the invention, the increase in caliper is determined by the recovery value as defined herein and the liquid acquisition layer has a recovery value at 45 ℃ of at least 65% of the recovery value at 20 ℃.

Drawings

While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, it is believed that the present invention will be better understood from the following drawings taken in conjunction with the accompanying description in which like elements are given the same reference number.

FIG. 1 is a top plan view of a disposable diaper with an upper layer partially cut away.

Fig. 2 is a cross-sectional view of the disposable diaper shown in fig. 1.

Detailed Description

Definition of

The following terms used herein have the following meanings:

"absorbent article" refers to a device that absorbs and contains liquid, and more specifically, refers to a device that is placed against or in proximity to the body of the wearer to absorb and contain the various exudates discharged from the body. Absorbent articles include, but are not limited to, diapers, adult incontinence briefs, training pants, diaper holders and liners, sanitary napkins, and the like. Absorbent articles also include wipes, such as household cleaning wipes, baby wipes, and the like.

The term "disposable" is used herein to describe articles that generally are not intended to be laundered or otherwise restored or reused i.e., the article is intended to be discarded after a single use and, preferably, to be recycled, composted or otherwise disposed of in an environmentally compatible manner.

"disposed" means that one element is formed (connected and positioned) in a particular place or position as a unitary structure with other elements or as a separate element joined to another element.

"diaper" refers to an absorbent article generally worn by infants and incontinent persons about the lower torso.

The terms "thickness" and "thick" are used interchangeably herein.

"attached" or "joined" includes configurations whereby an element is directly secured to another element by affixing the element directly to the other element; also included are configurations whereby an element is indirectly secured to another element by attaching one element to an intermediate member and then attaching the intermediate member to the other element.

"comprising" (in its verb noun and verb singular forms) is an inconclusive term that specifies the presence of, for example, one component described thereafter, but does not exclude other features, elements, steps or components that are known in the art or disclosed herein.

The term "hydrophilic" means that the fibers or surfaces of the fibers are wettable by aqueous fluids (e.g., aqueous body fluids) deposited on the fibers. Hydrophilicity and wettability are typically defined in terms of contact angle and fluid penetration time, for example, in terms of time to penetrate a nonwoven fabric. This is discussed in detail in a book entitled "Contact angle, website and edition" (copyright 1964) published by the American Chemical Society, Robert F.Gould. A fiber or surface of a fiber is said to be wetted by a fluid (i.e., hydrophilic) when either the contact angle between the fluid and the fiber or its surface is less than 90 deg., or the fluid tends to spread spontaneously across the surface of the fiber, both conditions generally co-exist. Conversely, a fiber or surface of a fiber is considered to be hydrophobic if the contact angle is greater than 90 ℃ and the fluid does not spread spontaneously across the surface of the fiber.

The terms "fiber" and "filament" are used interchangeably.

The terms "nonwoven", "nonwoven fabric" and "nonwoven web" are used interchangeably.

Absorbent article

FIG. 1 is a plan view of a diaper 20 which is a preferred embodiment of an absorbent article according to the present invention. The diaper is shown in its flat, uncontracted state (i.e., without elastic induced contraction). Portions of the structure are cut away to more clearly show the basic structure of the diaper 20. The portion of the diaper 20 that contacts the wearer faces the viewer. The chassis 22 of the diaper 20 in figure 1 comprises the main body of the diaper 20. The chassis 22 comprises an outer cover comprising a liquid pervious topsheet 24 and/or a liquid impervious backsheet 26. The chassis may also include most or all of the absorbent core 28 enclosed between the topsheet 24 and the backsheet 26. The chassis preferably further comprises side panels 30, leg cuffs 32 and a waist feature 34. The leg cuffs and waist feature typically comprise elastic members 33. One end portion of the diaper 20 is configured as the front waist region 36 of the diaper 20. The opposite end portion is configured as the back waist region 38 of the diaper 20. An intermediate portion of the diaper 20 is configured as a crotch region 37, which extends longitudinally between the front and back waist regions 36 and 38. The crotch region 37 is that portion of the diaper 20 which, when the diaper 20 is worn, is generally positioned between the wearer's legs. The waist regions 36 and 38 may comprise a fastening system comprising fastening members 40 preferably attached to the back waist region 38 and a landing zone 42 attached to the front waist region 36. The diaper 20 has a longitudinal axis 100 and a transverse axis 110. The periphery of the diaper 20 is defined by the outer edges of the diaper 20 in which the longitudinal edges 44 extend generally parallel to the longitudinal axis 100 of the diaper 20 and the end edges 46 extend generally parallel to the transverse axis 110 of the diaper 20.

For unitary absorbent articles, the chassis 22 comprises the primary structure of the diaper with the addition of other components to form a composite diaper structure. Although the topsheet 24, backsheet 26 and absorbent core 28 may be assembled in a variety of well-known configurations, preferred diaper configurations are generally described in U.S. patent 5,569,234 entitled "dispersible Pull-On Pant" issued 10/29 1996 to Buell et al; and U.S. patent 6,004,306 entitled "adsorbent arm Multi-orientation extension Side Panels" issued to Robles et al on 21.12.1999.

The topsheet 24 in fig. 1 may be fully or partially elasticized or may be foreshortened to provide a void space between the topsheet 24 and the absorbent core 28. Exemplary structures comprising elasticized or foreshortened topsheets are described in more detail in the following documents: U.S. Pat. No. 5,037,416, entitled "Disposable Absorbent Article Having elastic Extensible Topset", issued to Allen et al, 8/6, 1991; and U.S. Pat. No. 5,269,775 entitled "three sections Topsheets for Disposable adsorbent arms and Disposable adsorbent arms cutting Such Trisection Topsheets", issued on 12/14/1993 to Freeland et al.

The backsheet 26 in figure 1 is generally the portion of the diaper 20 disposed with the absorbent core 28 between the backsheet 26 and the topsheet 24. The backsheet 26 may be joined with the topsheet 24. The backsheet 26 prevents the exudates absorbed by the absorbent core 28 and contained within the article 20 from soiling other external articles that may contact the diaper 20, such as bed sheets and undergarments. In a preferred embodiment, the backsheet 26 is substantially impervious to liquids (e.g., urine) and comprises a nonwoven laminate and a thin plastic film, such as a thermoplastic film having a thickness of about 0.012mm (0.5mil) to about 0.051mm (2.0 mils). Suitable backsheet films include those manufactured by Tredegar Industries Inc. of Terre Haute, IN, and sold under the trade names X15306, X10962, and X10964. Other suitable backsheet materials may include breathable materials that permit vapors to escape from the diaper 20 while still preventing exudates from passing through the backsheet 26. Exemplary breathable materials may include materials such as woven webs, nonwoven webs, composite materials such as film-coated nonwoven webs, and microporous films such as, for example, ESPOIR NO, manufactured by Mitsui Toatsu co, japan, and EXXON chemical co, manufactured by EXXON chemical co, Bay City, tx.

The absorbent core 28 in figure 1 is generally disposed between the topsheet 24 and the backsheet 26. The absorbent core 28 may comprise any absorbent material that is generally compressible, conformable, non-irritating to the wearer's skin, and capable of absorbing and storing liquids such as urine and other certain body exudates. The absorbent core 28 may comprise a wide variety of liquid-absorbent materials commonly used in disposable diapers and other absorbent articles such as comminuted wood pulp, which is generally referred to as air felt. Other examples of suitable absorbent materials include creped cellulose wadding; melt blown polymers, including coformings; chemically stiffened, modified or cross-linked cellulosic fibers; tissue, including tissue wraps and tissue laminates, absorbent foams, absorbent sponges, superabsorbent polymers, absorbent gelling materials, or any other known absorbent material or combination of materials. The absorbent core may also comprise minor amounts (typically less than 10%) of non-liquid absorbent materials such as adhesives, waxes, oils and the like.

Exemplary Absorbent structures for use as Absorbent assemblies are described in U.S. Pat. No. 4,834,735 to Alemany et al, entitled "High Density Absorbent Members Having Low Density and Low Basis Weight Acquisition Zones", 1989, 30/5; and U.S. Pat. No. 5,625,222 entitled "Absorbent Foam Materials For aqueous fluids Made From high Internal Phase Emulsions Having Very high Water-To-Oil Ratios" issued To DesMarais et al on 7, 22/1997. .

The diaper 20 may also include such other features as are known in the art including front and back ear panels, waist cap panels, elastics and the like to provide better fit, storage and aesthetic characteristics. Such other features are well known in the art and are described in U.S. Pat. No. 3,860,003 to Buell et al entitled "controllable side relationships for dispersible diappers" at 1/14 1975 and U.S. Pat. No. 5,151,092 to Buell et al entitled "adsorbent aromatic with dynamic elastic bathing provision a predisposed reactive version change" at 29/9 1992.

In order to hold the diaper 20 in place about the wearer, the waist regions 36 and 38 may include a fastening system comprising fastening members 40 preferably attached to the back waist region 38. In a preferred embodiment, the fastening system further comprises a landing zone 42 attached to the front waist region 36. The fastening member is attached to the front waist region 36, preferably to the landing zone 42 to form leg openings and an article waist.

The diaper 20 according to the present invention may be provided with a reclosable fastening system or may alternatively be provided as a pant-type diaper.

The fastening system and any component thereof may comprise any material suitable for such use, including but not limited to plastics, films, foams, nonwoven webs, woven webs, paper, laminates, fiber reinforced plastics, and the like, or composites thereof. The material constituting the fastening device may preferably be a flexible material. The flexibility serves to enable the fastening system to conform to the body shape and thus reduces the likelihood that the fastening system will irritate or damage the skin of the wearer.

Fig. 2 shows a cross-sectional view of fig. 1 taken along a transverse axis 110. Starting from the wearer-facing side, the diaper comprises a topsheet 24, components of an absorbent core 28 and a backsheet 26. The absorbent core preferably comprises an acquisition system 50 comprising an upper acquisition layer 52 facing the wearer and a lower acquisition layer 54. In a preferred embodiment, the upper acquisition layer comprises a nonwoven fabric, while the lower acquisition layer preferably comprises a mixture of chemically stiffened, twisted and curled fibers, high surface area fibers and thermoplastic binding fibers. In another preferred embodiment both acquisition layers are preferably provided by a hydrophilic nonwoven material. The acquisition layer is preferably in direct contact with the storage layer 60.

An acquisition member according to the invention is preferably comprised by the acquisition system 50 of the diaper. Most preferably, the acquisition member is comprised by the upper acquisition layer 52. In a preferred embodiment of the present invention, the upper acquisition layer 52 consists of the claimed acquisition member.

The storage layer 60 is preferably coated with a core wrap material. In a preferred embodiment, the core wrap material comprises a top layer 56 and a bottom layer 58. The top layer 56 and the bottom layer 58 can be provided by a nonwoven material. One preferred material is a so-called SMS material comprising a spunbond layer, a meltblown layer and a further spunbond layer. The top layer 56 and the bottom layer 58 can be provided by two or more separate pieces of material or they can alternatively be provided by a unitary piece of material. Such a unitary sheet of material may be wrapped around the storage layer 60, for example, in a C-fold fashion. The top layer 56 and the bottom layer 58 may also be joined to one another, preferably along their peripheries. In a preferred option, the two layers are joined along their longitudinal peripheries, and in other embodiments, they are joined along a transverse periphery or along both longitudinal and transverse peripheries. Joining may be accomplished by a variety of methods well known in the art, such as by adhesive means, in a continuous or discontinuous pattern, and preferably in a linear or curvilinear pattern.

The storage layer 60 typically comprises fibrous material mixed with superabsorbent, absorbent gelling material. Other materials suitable for use as the absorbent core 28 described above may also be included.

Nonwoven fabric

Nonwoven fabrics are manufactured sheets, webs or batts of oriented or randomly oriented fibers bonded by friction and/or bonding and/or adhesion, or felted by wet milling, excluding paper and products which are woven, knitted, tufted, stitch-bonded incorporating binding yarns or filaments, whether or not additionally needled.

The source of the fibers may be natural or man-made. They may be staple or continuous or may be processed in situ.

Nonwoven fabrics can be made by a number of processes such as meltblowing, spunbonding, carding, and the like. The basis weight of nonwoven fabrics is typically expressed in grams per square meter (gsm).

Commercially available fibers range in diameter from less than about 0.001mm to over about 0.2mm, and they are marketed in several different forms: short fibers (referred to as chemical staple fibers or chopped fibers), continuous single fibers (filaments or monofilaments), untwisted continuous filament bundles (tows), and twisted continuous filament bundles (yarns). Fibers are classified according to their origin, chemical structure, or a combination of both. They can be woven into ropes and cords, made into felts (also known as nonwovens or non-woven fabrics), woven or knitted into textiles, or, in the case of high strength fibers, used as reinforcement in composites (that is, products made from two or more different materials).

The nonwoven fabric may comprise fibers made from naturally occurring fibers (natural fibers), from humans (synthetic or man-made), or composites thereof. Examples of natural fibers include, but are not limited to: animal fibers such as wool, silk, fur, and hair; plant fibers such as cellulose, cotton, flax and hemp; and certain naturally occurring mineral fibers. Synthetic fibers may be derived from natural fibers or non-natural fibers. Examples of synthetic fibers derived from natural fibers include, but are not limited to, rayon or lyocell fibers, both of which are derived from cellulose, a natural polysaccharide fiber. Synthetic fibers not derived from natural fibers may be derived from other natural sources or from mineral sources. Examples of synthetic fibers not derived from natural sources include, but are not limited to, polysaccharides such as starch. Examples of fibers derived from mineral sources include, but are not limited to, polyolefin fibers such as polypropylene, polyethylene fibers, and polyesters derived from petroleum, and silicate fibers such as glass and asbestos.

Nonwoven webs may be formed by direct extrusion processes during which the fibers and web are formed at about the same point in time; or by preformed fibers that can be laid into the web at a significantly delayed point in time. Examples of direct extrusion processes include, but are not limited to: typically layer forming spunbond, meltblown, solution spun, electrospun, and combinations thereof.

Examples of the web forming method include wet-forming and dry-forming. Exemplary dry-laid processes include, but are not limited to, air-laying, carding, and combinations thereof, which typically form a layer. The combination of the above methods produces a nonwoven material generally referred to as a mixture or composite. Examples of combinations include, but are not limited to, spunbond-meltblown-spunbond (SMS), spunbond-carded (SC), spunbond-air-Stream (SA), meltblown-air-stream (MA), and combinations thereof, typically in layers. Combinations including direct extrusion can be combined with the direct extrusion process at about the same point in time (e.g., spin forming and co-forming of SA and MA), or at a subsequent point in time. In the above embodiments, one or more monolayers may be produced by each method. For example, SMS may mean a three-layer "SMS" web, a five-layer "ssmms" web, or any suitable variation thereof, wherein lower case letters indicate individual layers and upper case letters indicate a general designation of similar adjacent layers.

The fibers in the nonwoven web are typically joined to one or more adjacent fibers at some overlapping juncture. This includes joining fibers within each layer, and joining fibers between layers when there is more than one layer. The fibers may be joined by mechanical entanglement, by chemical bonding, or by a combination thereof. The fibers may also be joined by thermal bonding, which includes techniques such as through-air bonding and thermal bonding using heated press rolls.

All of the above-described fibers and processing techniques can be used to provide an acquisition member in accordance with the present invention.

Preferred acquisition Member

Any material comprising a plurality of fibers and a binder, i.e. any nonwoven material as described above, is suitable for providing a liquid acquisition layer according to the present invention. The corresponding material must have good recovery in connection with the application and removal of external pressure. In particular, the materials have improved recovery at elevated temperatures compared to prior art materials.

A preferred material will have good recovery at 20 c, which is considered to be representative of room temperature and therefore also of many transport and storage conditions. A preferred material will also have good recovery values over a large temperature range (i.e. temperatures above 20 c). A temperature of 45 c is considered to represent a higher temperature storage condition, but this temperature is also considered to represent an upper temperature limit encountered under wearing conditions. The temperature of 60 c is believed to represent the more extreme storage and transport conditions encountered in particularly high climatic conditions. Acquisition layers according to the present invention will not significantly lose their ability to recover from external pressure when the material is kept at a temperature of 45 ℃ or 60 ℃ respectively.

Preferred materials according to the present invention will recover at least 70%, more preferably at least 75% or 80% at 20 ℃. For preferred materials, the recovery value at 45 ℃ will be at least 45%, more preferably 50%, 55% or at least 60%.

According to the invention, the recovery value at 45 ℃ is at least 65% of the recovery value at 20 ℃. More preferred is a higher recovery value, i.e. 70%, more preferably 75%, still more preferably 80% of the recovery value at 20 ℃. Preferred materials will also still have a high recovery value at 60 ℃ compared to their recovery value at 20 ℃, preferably a recovery value at 60 ℃ of at least 50%, more preferably 55%, still more preferably 60% or 65% of the recovery value at 20 ℃.

The present range of material recovery values for absorbent articles and liquid acquisition layers of such absorbent articles are related to different pressures. For example, a seated infant will exert a pressure of about 2.1kPa (about 0.3psi), depending on which pressure, however, is highly dependent on the infant wearing the diaper and its sitting environment. Absorbent articles such as diapers are typically subjected to pressures in the range of 6 to 10kPa (about 0.9 to 1.5psi) when they are shipped in plastic packaging. When materials for liquid acquisition layers are transported in rolls, they may be subjected to pressures of up to 50kPa (about 7 psi). Without wishing to be bound by theory, it is believed that the recovery characteristics of a material at very high pressures also represent the recovery value at lower pressures. Thus, it is believed that materials which provide excellent recovery properties at high pressures, as experienced for example in roll shipping, will also ensure excellent recovery when the package is removed and when worn. Therefore, the recovery value was measured at a pressure of 50kPa as described below.

The liquid acquisition layer according to the present invention comprises a plurality of fibers, preferably in the form of a nonwoven material and a binder. Although many fibers and binders are suitable, it has been found that certain materials provide better recovery than others and are therefore preferred for the materials according to the present invention.

Preferred fibers according to the invention are polyester fibers, such as PET fibers. It has been found that the acquisition material works best if a blend of different fibers is used. While blends of 3, 4, 5 or more different fibers may be used, it is preferred to use a blend of two fibers. Such blends may include at least 10, 20, 30, 40, 50, 60, 70, 80, or 90 weight percent of the first fiber with the remainder being the weight percent of the second fiber. Highly preferred according to the present invention is a 50: 50 blend of fibers, i.e., 50% by weight of a first fiber is mixed with 50% by weight of a second fiber.

Preferably, at least the second type of fibers will exhibit a helical crimp. As used herein, spiral crimp is any three-dimensional crimp and preferably wherein one of the fibers substantially assumes a helical shape.

Optionally, the first fiber may also be helically crimped. In one embodiment of the invention, the first fiber and the second fiber are identical.

The first fibers may be finer than the second fibers. Preferably, the first fiber will have a fiber length of 3 to 9dtex, more preferably 5 to 8dtex, most preferably 6 to 7 dtex. The second fiber will preferably have a fiber length of 8-12, more preferably 9-10 dtex. The first and second fibers may have different lengths, however preferably the same length. Preferably, the average length of the fibres is from 20 to 70mm, more preferably from 30 to 50 mm. The first type of fiber will preferably have a higher number of crimps than the second type of fiber. The preferred number of crimps for the third fiber is 8-12 crimps per inch (cpi), more preferably 9-10 cpi. For the second fiber, 4-8cpi is preferred, 5-7cpi is even more preferred.

A preferred first fiber is available from Wellman, inc. A preferred second fiber is available from Wellman, inc. under the trade designation T5974.

One preferred type of fiber for use in the present invention is a so-called bicomponent fiber, wherein one fiber is provided by different materials, typically a first and a second polymeric material. The two materials may be chemically different (so the fibers are chemically dissimilar) or they may differ only in their physical properties (so the fibers are chemically homogeneous) while being chemically homogeneous. For example, the intrinsic viscosities of the two materials are different and have been found to affect the crimp properties of the bicomponent fibers. Thus, chemically anisotropic bicomponent fibers and chemically homogeneous bicomponent fibers are preferred in accordance with the present invention.

Bicomponent fibers which are particularly suitable for the second fiber are side-by-side bicomponent fibers, as disclosed in WO 99/00098. One particularly preferred type of bicomponent fiber is a circular cross-section fiber having a hollow space in the center. It is preferred that 10-15% of the cross-sectional area is hollow, and more preferably 20-30% of the cross-sectional area is hollow.

Preferably, at least one fiber and preferably two or more fibers are crimped. Two-dimensional or "planar crimping" is preferred for the first type of fiber. Three-dimensional or spiral crimp is preferred for the second fiber. It is believed that the use of side-by-side bicomponent fibers is beneficial for imparting spiral crimp to the fibers.

Without wishing to be bound by theory, it is also believed that the helical crimp of the fibers is very beneficial for their liquid acquisition and spreading properties. It is assumed that the helical crimp increases the void space in acquisition members formed from such fibers. Typically, absorbent articles are exposed to certain pressures exerted by the wearer when worn, which may reduce void space in the acquisition member. Having good permeability and sufficient available void space is important for good liquid diffusion and transport. It is also believed that bicomponent helically crimped fibers as described above are well suited to maintaining sufficient void volume even when the acquisition member is exposed to pressure. Helically crimped fibers are also believed to provide good permeability for a given fiber dtex value, with a hollow fiber cross-section allowing for a larger fiber outer diameter than a dense cross-section. The outer diameter of the fibers appears to determine the permeability properties of acquisition members formed from such fibers.

Although any of the above-described nonwoven processing methods are suitable for providing an acquisition member according to the present invention, one highly preferred processing method for such an acquisition member is the carded resin bonding technique as described in WO 98/22279. As part of this process, the adhesive will be cured and dried after it has been applied to the web.

It is preferred that the nonwoven material has a basis weight of from 20 to 100, preferably from 30 to 80, and most preferably from 50 to 70 grams per square meter. Basis weight was determined using a 100cm long sample with a width given by the roll width (however at least 10 cm).

The material of the invention is preferably stabilized with a latex binder and most preferably a styrene-butadiene latex binder (SB latex). Methods for obtaining such latices are known, for example from EP 149880 (Kwok) and US 2003/0105190(Diehl et al). However, preferred SB latexes are obtained using more than 10% by weight of mono-or dicarboxylic acids, and will be referred to herein as having a carboxylation level of 10% or more. Preferred SB latexes will have a carboxylation level of 10% to 25%, more preferably 10% to 20%. Depending on the final application, adhesive levels in excess of 12%, 14% or 16% are preferred. An especially preferred SB latex is available under the trade name GENFLOTM 3160 (OMNOVA Solutions Inc. of Akron, Ohio).

Without being bound by theory, it is believed that such adhesives are more effective because the temperature sensitivity of the recovery value of such adhesives is reduced compared to other adhesives of the prior art. As is well known, latex adhesives are applied to nonwoven structures in liquid form and accumulate at the fiber intersections. After curing, the binder forms an associated polymer film that stabilizes the nonwoven structure. When the nonwoven material is placed under a force (e.g., by compression), the polymer film resists that stress so that the fibers that make up the nonwoven material cannot move. If the stress causes the polymer film to extend beyond its yield value, the fibers may move irreversibly, resulting in reduced recovery. The applicant believes that the recovery value of the liquid acquisition layer of the present invention is less temperature sensitive, as the binder employed has an improved rheological behaviour at high temperatures, i.e. an improved elastic behaviour. Thus, the liquid acquisition layer of the present invention may recover from applied stress, rather than yield to produce irreversible fiber movement.

In accordance with the present invention, a film formed from the adhesive material has a tan delta value at 40 ℃ that is not greater than the tan delta value at 20 ℃.

Tan δ values were obtained according to the test methods described herein. Preferred liquid acquisition layers according to the present invention have a tan delta value at 40 of less than 0.6, preferably less than 0.5, more preferably less than 0.4, most preferably less than 0.3.

Without wishing to be bound by theory, it appears that the tan delta value increases with temperature, resulting in a loss of volume of the liquid acquisition layer when the member is under high temperature external pressure. When they have been subjected to pressure at elevated temperatures, it appears that the liquid acquisition members will generally not recover nor acquire their initial volume, and that at such a temperature their tan delta value is higher than at lower temperatures.

Furthermore, without wishing to be bound by theory, it appears to be beneficial for the film comprised of the adhesive for the liquid acquisition member to have a certain G' -curve. In an important aspect, the G' versus temperature curve should have an elongated rubbery plateau. The elongated rubbery plateau corresponds to a relatively constant G' value over a wide temperature range. Preferably, G' is constant over a temperature range of at least 0 ℃ to 80 ℃, preferably at least 0 ℃ to 60 ℃, most preferably at least 20 ℃ to 40 ℃. In this context, G 'is considered constant if the maximum value of G' and its minimum value within a given range differ by no more than 15%, preferably no more than 10%, most preferably no more than 5%.

Applicants believe, in particular, that the increase in carboxylation levels compared to prior art nonwoven adhesives provides a G' curve with an elongated rubbery plateau. Adhesives with extended rubbery plateaus remain elastic until higher temperatures. Thus, nonwoven materials incorporating such binders can yield with external pressure, but without substantial rearrangement in their fibrous structure. The applicant believes that this contributes to their better resilience.

In another important aspect, it is believed that the adhesive materials that must be used in the present invention will have a low glass transition temperature, Tg. The glass transition temperature as used herein is measured by Differential Scanning Calorimetry (DSC). Preferably, the Tg is in the range of-10 ℃ to +10 ℃, more preferably in the range of-5 ℃ to +5 ℃, most preferably-3 ℃ to +1 ℃. The glass transition temperature Tg well below the intended use and storage temperature of the liquid acquisition layer helps to maintain the rubber-like properties of the adhesive and thus improves the ability of the layers to recover. Because the present invention preferably employs adhesives having an extended rubbery plateau, the adhesive may have a low glass transition temperature Tg and at the same time be capable of functioning as an adhesive in the rubbery plateau state at elevated temperatures, e.g., 40 ℃, 60 ℃, or 80 ℃.

Test method

Determination of recovery value

Equipment: a combination of Dynamic Mechanical Analyzer 2980 from TA Instruments, USA, and Gas Cooking Access (GCA), or similar device. The GCA uses liquid nitrogen which is vaporized and directed through the temperature-controlled chamber of the DMA to control the temperature in conjunction with the heaters of the temperature-controlled chamber and provide an inert measurement atmosphere. The constant temperature stability should not be less than +/-1 ℃.

For operation in compression mode, a standard compression jig supplied by TA Instruments with a compression plate diameter of 15mm was used. The maximum compression force of the instrument should be 18N or greater. The optical encoder (position sensor determining the position of the drive shaft, which is attached to the movable upper compression plate) delivers a strain resolution of 1 nm.

The method comprises the following steps:

sample preparation:

a cylindrical sample of 13mm diameter was punched out of the nonwoven sample (NW) using a punch supplied by TA Instruments and placed on the lower plate of the compression jig. To determine the initial thickness of the NW, the upper plate was carefully placed on the NW by hand. The upper plate should not contact the NW in order to prevent uncontrolled precompression. After the upper plate is properly placed, the motor drive shaft is deactivated to hold the upper plate in the proper position.

DMA-compressive creep test method:

the compressive creep test consists of a compression step that applies a certain stress to the NW followed by a recovery step that sets the stress to zero in order to determine the resilience or recoverability of the NW.

The following process parameters are defined:

temperature: the temperature was kept constant for the whole experiment. Experiments are typically performed at either 20 ℃, 45 ℃ or 60 ℃ to determine the temperature dependence of the recovery function. (one experiment can be done for each temperature option.)

The balance time is as follows: an equilibration time of at least 10 minutes must be scheduled before the compression step in order for the NW and the instrument to reach thermal equilibrium.

Poisson ratio: 0.44

Preloading force: 0.02N, applied throughout the experiment to keep the upper compression plate in contact with the NW samples after the method was started.

And (3) sample determination: cylindrical:

diameter: 13mm, determined by punch

Thickness: to ensure the pre-compression defined, a force of 0.02N was applied to the upper plate for 5 seconds, holding the upper plate against the NW, for all experiments, before determining the sample thickness. (start method and cancel after 5 seconds, stop drive shaft after cancel compression should reach equilibrium within 20 microns of the upper plate.) equilibrium thickness is obtained. The distance between the lower and upper clamp plates determines the sample thickness (digital thickness, 2 digits).

Compression step time: 120min

Applied stress: 50kPa (about 7psi)

The DMA uses a clamp correction factor to calculate the lateral strain due to the expansion effect in compression mode. The stress needs to be input to get the actual stress on the material to be 50kPa (about 7psi) depending on the sample thickness taking into account the clamp factor. The stress was calculated using the corresponding fixture factor multiplied by 50kPa (about 7psi) at a given sample thickness (including the table of sample thickness versus fixture factor provided by TA Instruments). The fixture factor must be inserted to match the sample thickness.

Recovery step time: 120min

Applied stress: 0Pa

Reporting:

the recovery value in% (recovered sample thickness/initial sample thickness) x 100 versus recovery time is plotted against the initial sample thickness. The final recovery value after 120min was recorded as a function of the measured temperature.

Tan delta measurement

The tan delta value of an adhesive comprising a polymer dispersion in a substantially aqueous medium is measured on a polymer film prepared from the adhesive. To this end, about 2% of the Paraglum 147 thickener was added and mixed well with the dispersion and allowed to stand for about 15 min. The film is then drawn off using a film doctor blade and the essentially aqueous medium is evaporated in a ventilated oven at 50 ℃ and allowed to dry further overnight until it is removed. Subsequently, the cured product was cured at 150 ℃ for 60 seconds.

The thickness of the respective film is not critical. The measurement was carried out very well on films with a thickness of 0.3 mm.

Tan δ is a rheological property well known in the art that can be conveniently measured using a variety of equipment. Although tan δ corresponds to the viscous modulus G "divided by the elastic modulus G ', tan δ values can also be obtained without first obtaining G' and G" by using standard rheological equipment. Mechanical Analyzer 2980 supplied by TAInstructions, USA, in combination with Gas cooking Access has been found to work well for such measurements.

Tan δ values as used herein are those measured in the stretch mode.

Claims (12)

1. A liquid acquisition layer for an absorbent article, the liquid acquisition layer comprising a plurality of fibers and a binder, the liquid acquisition layer having a thickness, the thickness decreasing with external pressure and the thickness increasing after the external pressure is removed, the increase in thickness being determined by the recovery value as defined herein, the liquid acquisition layer having a recovery value at 20 ℃ and a recovery value at 45 ℃,

is characterized in that:

the recovery value at 45 ℃ is at least 65% of the recovery value at 20 ℃.

2. A liquid acquisition layer according to claim 1, wherein said recovery value at 45 ℃ is at least 70%, more preferably at least 75%, still more preferably 80% of said recovery value at 20 ℃.

3. A liquid acquisition layer according to claim 1 or 2, wherein the binder comprises a styrene-butadiene latex binder.

4. A liquid acquisition layer according to claim 3, wherein the styrene-butadiene latex binder has a carboxylation level of at least 10%, preferably at least 12%.

5. A liquid acquisition layer according to any of the preceding claims, wherein said fibers are polyester fibers and said liquid acquisition layer comprises from 20 to 40 wt% of said binder and from 60 to 80 wt% of said polyester fibers.

6. A liquid acquisition layer according to claim 5 wherein said polyester fibers comprise from 20 to 80% by weight of first fibers and from 20 to 80% by weight of second fibers, said second fibers comprising spirally crimped fibers.

7. A liquid acquisition layer according to claim 6, wherein said first fibers exhibit a planar curl.

8. A liquid acquisition layer according to claim 6 wherein said second fibers comprise hollow, chemically homogeneous bicomponent fibers.

9. A liquid acquisition layer according to any of the preceding claims, wherein the polyester fibers are carded to form a nonwoven material.

10. An absorbent article comprising an acquisition member according to any of the preceding claims, such as a diaper and an incontinence pad, a sanitary napkin, a panty liner or the like.

11. An absorbent article comprising the acquisition member of any of the preceding claims, the absorbent article further comprising a layer of chemically stiffened cellulosic fibers.

12. An absorbent article according to claim 11, said article comprising a topsheet and an absorbent core, said topsheet facing the wearer when said article is in an intended wearing position, said liquid acquisition layer and said layer of chemically stiffened cellulosic fibers being positioned between said topsheet and said absorbent core.